Quorum sensing is a process by which bacteria coordinate gone expression through the secretion and sensing of chemical signals. Vibrio harveyi has become a model system for quorum sensing, as it possesses both an intra- and inter-species signaling systems. Although much of the V. harveyi quorum sensing system is well understood, two major questions remain. This proposal seeks to answer those questions. First, the putative repressor of LuxR, the transcriptional activator of the luminescence operon, induced by LuxO in low cell density will be identified. Based on preliminary results, the repressor is hypothesized to be a complex of Hfq and a small, regulatory RNA (sRNA). The sRNA will be identified by screening a plasmid library of V. harveyi genomic DNA for the ability to repress constitutive light production. Secondly, the regulon of quorum sensing in V. harveyi remains undefined. Specifically, it is unclear if V. harveyi is able to regulate specific sets of genes in response to the intra- or inter-species signals alone or in combination with one another. Using differential fluorescence induction analysis, the gone targets regulated in the different autoinducer input states will be determined and analyzed. These studies will further our understanding of how V. harveyi senses, and responds to, different environmental conditions. Importantly, as quorum sensing has been shown to regulate virulence in a number of different bacterial pathogens, these finding will be directly applicable to the understanding, and prevention, of disease.